Invalid postback or callback argument.

Hello,
 
I am not sure I really understand this error problem. One minute my Web App was working another minute I get this:
I have a GridView that has ImageButtons in Itemtemplates. When I click any of the ImageButtons I get the below error message. In my WebConf file and Page source I did not see anything related to EnableEventValidation = true

 
Invalid postback or callback argument.  Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%@ Page EnableEventValidation="true" %> in a page.  For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them.  If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation.
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